soc: aspeed-lpc-ctrl: LPC to AHB mapping on ast2600

The ast2600 disables the mapping of AHB memory regions by default,
only allowing the LPC window to point to SPI NOR. In order to point the
window to any AHB address, an ast2600 specific bit must be toggled.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200312121413.294384-1-joel@jms.id.au


Signed-off-by: Joel Stanley <joel@jms.id.au>